Comments

Enable MMIOTRACE to allow us to trace low level card operations. This is
commonly used to debug graphics issues particularly under Nouveau.
According to the documentation it is a near zero impact when disabled.
Signed-off-by: Andy Whitcroft <apw@canonical.com>
---
debian.master/config/config.common.ubuntu | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)

On Wed, Mar 17, 2010 at 03:03:41AM -0700, Bryce Harrington wrote:
> On Wed, Mar 17, 2010 at 09:26:24AM +0000, Andy Whitcroft wrote:> > Enable MMIOTRACE to allow us to trace low level card operations. This is> > commonly used to debug graphics issues particularly under Nouveau.> > According to the documentation it is a near zero impact when disabled.> > "when enabled" I hope?
The cost is near-zero for having the MMIOTRACE option enabled with the
trace collection turned off (the default). We take one additional branch
in the mmio map/upmap paths which are non-critical initialisation/teardone
paths in normal usage. The actual mmio operations are not affected at all.
When we enable trace collection the overhead is high, taking a page
fault for each access to the mmio mapping.
-apw

On Wed, 2010-03-17 at 03:03 -0700, Bryce Harrington wrote:
> On Wed, Mar 17, 2010 at 09:26:24AM +0000, Andy Whitcroft wrote:> > Enable MMIOTRACE to allow us to trace low level card operations. This is> > commonly used to debug graphics issues particularly under Nouveau.> > According to the documentation it is a near zero impact when disabled.> > "when enabled" I hope?>
No; when it's enabled it's a huge performance hit. It's not uncommon
for a mmio-traced nvidia blob to take multiple minutes to bring up X.

On Thu, 2010-03-18 at 16:43 +1100, Christopher James Halse Rogers wrote:
> On Wed, 2010-03-17 at 03:03 -0700, Bryce Harrington wrote:> > On Wed, Mar 17, 2010 at 09:26:24AM +0000, Andy Whitcroft wrote:> > > Enable MMIOTRACE to allow us to trace low level card operations. This is> > > commonly used to debug graphics issues particularly under Nouveau.> > > According to the documentation it is a near zero impact when disabled.> > > > "when enabled" I hope?> > > No; when it's enabled it's a huge performance hit. It's not uncommon> for a mmio-traced nvidia blob to take multiple minutes to bring up X.>
To be clear here:
The kernel config option, when enabled, adds a new available tracer,
which when itself is enabled, causes a huge performance hit.
Enabling the kernel config option doesn't cause a huge performance hit
in its own right.
Scott